Ammonoid recovery after the Permian–Triassic mass extinction: a re-exploration of morphological and phylogenetic diversity patterns
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The explosive ammonoid rediversification after the Permian–Triassic mass extinction is now well understood in terms of taxonomic richness and biogeography. Using an updated dataset of Early Triassic ammonoids, we compare morphological disparity and taxonomic richness patterns at the regional and global scales. Disparity evolved similarly at both scales, suggesting a global influence of abiotic factors. Morphological diversification occurred early in the Smithian and a marked contraction of the morphospace took place during the end-Smithian extinction. We confirm that trends in disparity and richness were decoupled during the Griesbachian and Dienerian. Three macroevolutionary processes may be involved: (1) a nonselective extinction at the Permian–Triassic boundary; (2) a Dienerian constrained radiation with several homeomorphic genera; (3) a potential deterministic extinction during the end-Smithian crisis. We also demonstrate a superfamily imprint upon disparity for the Spathian when most superfamilies occupied a restricted part of the morphospace. Sphaerocones were the most affected by the Dienerian and end-Smithian extinction, but explanations remain elusive. On the one hand, this may be linked to widespread harsh conditions at those times. On the other hand, as the sphaerocones occurred episodically during the Early Triassic, this might be explained by a relaxing of ecological constraints or simply by convergent evolution.